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package java.util;

import java.security.*;

/**
 * A class that represents an immutable universally unique identifier (UUID).
 * A UUID represents a 128-bit value.
 *
 * <p> There exist different variants of these global identifiers.  The methods
 * of this class are for manipulating the Leach-Salz variant, although the
 * constructors allow the creation of any variant of UUID (described below).
 *
 * <p> The layout of a variant 2 (Leach-Salz) UUID is as follows:
 *
 * The most significant long consists of the following unsigned fields:
 * <pre>
 * 0xFFFFFFFF00000000 time_low
 * 0x00000000FFFF0000 time_mid
 * 0x000000000000F000 version
 * 0x0000000000000FFF time_hi
 * </pre>
 * The least significant long consists of the following unsigned fields:
 * <pre>
 * 0xC000000000000000 variant
 * 0x3FFF000000000000 clock_seq
 * 0x0000FFFFFFFFFFFF node
 * </pre>
 *
 * <p> The variant field contains a value which identifies the layout of the
 * {@code UUID}.  The bit layout described above is valid only for a {@code
 * UUID} with a variant value of 2, which indicates the Leach-Salz variant.
 *
 * <p> The version field holds a value that describes the type of this {@code
 * UUID}.  There are four different basic types of UUIDs: time-based, DCE
 * security, name-based, and randomly generated UUIDs.  These types have a
 * version value of 1, 2, 3 and 4, respectively.
 *
 * <p> For more information including algorithms used to create {@code UUID}s,
 * see <a href="http://www.ietf.org/rfc/rfc4122.txt"> <i>RFC&nbsp;4122: A
 * Universally Unique IDentifier (UUID) URN Namespace</i></a>, section 4.2
 * &quot;Algorithms for Creating a Time-Based UUID&quot;.
 *
 * <p>
 *  表示不可变的通用唯一标识符(UUID)的类。 UUID表示128位值。
 * 
 *  <p>这些全局标识符存在不同的变体。该类的方法用于操作Leach-Salz变体,尽管构造函数允许创建UUID的任何变体(如下所述)。
 * 
 *  <p>变体2(Leach-Salz)UUID的布局如下：
 * 
 *  最重要的长度包括以下无符号字段：
 * <pre>
 *  0xFFFFFFFF00000000 time_low 0x00000000FFFF0000 time_mid 0x000000000000F000 version 0x0000000000000FF
 * F time_hi。
 * </pre>
 *  最低有效长度由以下无符号字段组成：
 * <pre>
 *  节点0xC000000000000000变量0x3FFF000000000000 clock_seq 0x0000FFFFFFFFFFFF
 * </pre>
 * 
 *  <p> variant字段包含一个标识{@code UUID}的布局的值。上述位布局仅适用于变量值为2的{@code UUID},表示Leach-Salz变体。
 * 
 *  <p>版本字段包含描述此{@code UUID}类型的值。有四种不同的基本类型的UUID：基于时间,DCE安全,基于名称和随机生成的UUID。这些类型的版本值分别为1,2,3和4。
 * 
 * <p>有关包括用于创建{@code UUID}的算法的详细信息,请参见<a href="http://www.ietf.org/rfc/rfc4122.txt"> <i> RFC 4122：通用唯一标识
 * 符(UUID)URN命名空间</span> </a>,第4.2节"用于创建基于时间的UUID的算法"。
 * 
 * 
 * @since   1.5
 */
public final class UUID implements java.io.Serializable, Comparable<UUID> {

    /**
     * Explicit serialVersionUID for interoperability.
     * <p>
     *  显式serialVersionUID用于互操作性。
     * 
     */
    private static final long serialVersionUID = -4856846361193249489L;

    /*
     * The most significant 64 bits of this UUID.
     *
     * <p>
     *  此UUID的最高有效64位。
     * 
     * 
     * @serial
     */
    private final long mostSigBits;

    /*
     * The least significant 64 bits of this UUID.
     *
     * <p>
     *  此UUID的最低有效64位。
     * 
     * 
     * @serial
     */
    private final long leastSigBits;

    /*
     * The random number generator used by this class to create random
     * based UUIDs. In a holder class to defer initialization until needed.
     * <p>
     *  这个类用来创建基于随机的UUID的随机数生成器。在持有者类中推迟初始化直到需要。
     * 
     */
    private static class Holder {
        static final SecureRandom numberGenerator = new SecureRandom();
    }

    // Constructors and Factories

    /*
     * Private constructor which uses a byte array to construct the new UUID.
     * <p>
     *  私有构造函数使用一个字节数组来构造新的UUID。
     * 
     */
    private UUID(byte[] data) {
        long msb = 0;
        long lsb = 0;
        assert data.length == 16 : "data must be 16 bytes in length";
        for (int i=0; i<8; i++)
            msb = (msb << 8) | (data[i] & 0xff);
        for (int i=8; i<16; i++)
            lsb = (lsb << 8) | (data[i] & 0xff);
        this.mostSigBits = msb;
        this.leastSigBits = lsb;
    }

    /**
     * Constructs a new {@code UUID} using the specified data.  {@code
     * mostSigBits} is used for the most significant 64 bits of the {@code
     * UUID} and {@code leastSigBits} becomes the least significant 64 bits of
     * the {@code UUID}.
     *
     * <p>
     *  使用指定的数据构造新的{@code UUID}。
     *  {@code mostSigBits}用于{@code UUID}的最高有效64位,{@code leastSigBits}成为{@code UUID}的最低有效64位。
     * 
     * 
     * @param  mostSigBits
     *         The most significant bits of the {@code UUID}
     *
     * @param  leastSigBits
     *         The least significant bits of the {@code UUID}
     */
    public UUID(long mostSigBits, long leastSigBits) {
        this.mostSigBits = mostSigBits;
        this.leastSigBits = leastSigBits;
    }

    /**
     * Static factory to retrieve a type 4 (pseudo randomly generated) UUID.
     *
     * The {@code UUID} is generated using a cryptographically strong pseudo
     * random number generator.
     *
     * <p>
     *  静态工厂检索类型4(伪随机生成)的UUID。
     * 
     *  {@code UUID}是使用密码强的伪随机数生成器生成的。
     * 
     * 
     * @return  A randomly generated {@code UUID}
     */
    public static UUID randomUUID() {
        SecureRandom ng = Holder.numberGenerator;

        byte[] randomBytes = new byte[16];
        ng.nextBytes(randomBytes);
        randomBytes[6]  &= 0x0f;  /* clear version        */
        randomBytes[6]  |= 0x40;  /* set to version 4     */
        randomBytes[8]  &= 0x3f;  /* clear variant        */
        randomBytes[8]  |= 0x80;  /* set to IETF variant  */
        return new UUID(randomBytes);
    }

    /**
     * Static factory to retrieve a type 3 (name based) {@code UUID} based on
     * the specified byte array.
     *
     * <p>
     *  静态工厂根据指定的字节数组检索类型3(基于名称){@code UUID}。
     * 
     * 
     * @param  name
     *         A byte array to be used to construct a {@code UUID}
     *
     * @return  A {@code UUID} generated from the specified array
     */
    public static UUID nameUUIDFromBytes(byte[] name) {
        MessageDigest md;
        try {
            md = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException nsae) {
            throw new InternalError("MD5 not supported", nsae);
        }
        byte[] md5Bytes = md.digest(name);
        md5Bytes[6]  &= 0x0f;  /* clear version        */
        md5Bytes[6]  |= 0x30;  /* set to version 3     */
        md5Bytes[8]  &= 0x3f;  /* clear variant        */
        md5Bytes[8]  |= 0x80;  /* set to IETF variant  */
        return new UUID(md5Bytes);
    }

    /**
     * Creates a {@code UUID} from the string standard representation as
     * described in the {@link #toString} method.
     *
     * <p>
     *  从{@link #toString}方法中描述的字符串标准表示创建{@code UUID}。
     * 
     * 
     * @param  name
     *         A string that specifies a {@code UUID}
     *
     * @return  A {@code UUID} with the specified value
     *
     * @throws  IllegalArgumentException
     *          If name does not conform to the string representation as
     *          described in {@link #toString}
     *
     */
    public static UUID fromString(String name) {
        String[] components = name.split("-");
        if (components.length != 5)
            throw new IllegalArgumentException("Invalid UUID string: "+name);
        for (int i=0; i<5; i++)
            components[i] = "0x"+components[i];

        long mostSigBits = Long.decode(components[0]).longValue();
        mostSigBits <<= 16;
        mostSigBits |= Long.decode(components[1]).longValue();
        mostSigBits <<= 16;
        mostSigBits |= Long.decode(components[2]).longValue();

        long leastSigBits = Long.decode(components[3]).longValue();
        leastSigBits <<= 48;
        leastSigBits |= Long.decode(components[4]).longValue();

        return new UUID(mostSigBits, leastSigBits);
    }

    // Field Accessor Methods

    /**
     * Returns the least significant 64 bits of this UUID's 128 bit value.
     *
     * <p>
     *  返回此UUID的128位值的最低有效64位。
     * 
     * 
     * @return  The least significant 64 bits of this UUID's 128 bit value
     */
    public long getLeastSignificantBits() {
        return leastSigBits;
    }

    /**
     * Returns the most significant 64 bits of this UUID's 128 bit value.
     *
     * <p>
     *  返回此UUID的128位值的最高有效64位。
     * 
     * 
     * @return  The most significant 64 bits of this UUID's 128 bit value
     */
    public long getMostSignificantBits() {
        return mostSigBits;
    }

    /**
     * The version number associated with this {@code UUID}.  The version
     * number describes how this {@code UUID} was generated.
     *
     * The version number has the following meaning:
     * <ul>
     * <li>1    Time-based UUID
     * <li>2    DCE security UUID
     * <li>3    Name-based UUID
     * <li>4    Randomly generated UUID
     * </ul>
     *
     * <p>
     *  与此{@code UUID}相关联的版本号。版本号说明如何生成此{@code UUID}。
     * 
     * 版本号具有以下含义：
     * <ul>
     *  <li> 1基于时间的UUID <li> 2 DCE安全性UUID <li> 3基于名称的UUID <li> 4随机生成的UUID
     * </ul>
     * 
     * 
     * @return  The version number of this {@code UUID}
     */
    public int version() {
        // Version is bits masked by 0x000000000000F000 in MS long
        return (int)((mostSigBits >> 12) & 0x0f);
    }

    /**
     * The variant number associated with this {@code UUID}.  The variant
     * number describes the layout of the {@code UUID}.
     *
     * The variant number has the following meaning:
     * <ul>
     * <li>0    Reserved for NCS backward compatibility
     * <li>2    <a href="http://www.ietf.org/rfc/rfc4122.txt">IETF&nbsp;RFC&nbsp;4122</a>
     * (Leach-Salz), used by this class
     * <li>6    Reserved, Microsoft Corporation backward compatibility
     * <li>7    Reserved for future definition
     * </ul>
     *
     * <p>
     *  与此{@code UUID}相关联的变体号。变体编号描述{@code UUID}的布局。
     * 
     *  变体号具有以下含义：
     * <ul>
     *  <li> 0保留用于NCS向后兼容性<li> 2 <a href="http://www.ietf.org/rfc/rfc4122.txt"> IETF&nbsp; RFC&nbsp; 4122 </a>
     * (Leach-Salz),由此类使用<li> 6保留,Microsoft Corporation向后兼容性<li> 7保留以供将来使用。
     * </ul>
     * 
     * 
     * @return  The variant number of this {@code UUID}
     */
    public int variant() {
        // This field is composed of a varying number of bits.
        // 0    -    -    Reserved for NCS backward compatibility
        // 1    0    -    The IETF aka Leach-Salz variant (used by this class)
        // 1    1    0    Reserved, Microsoft backward compatibility
        // 1    1    1    Reserved for future definition.
        return (int) ((leastSigBits >>> (64 - (leastSigBits >>> 62)))
                      & (leastSigBits >> 63));
    }

    /**
     * The timestamp value associated with this UUID.
     *
     * <p> The 60 bit timestamp value is constructed from the time_low,
     * time_mid, and time_hi fields of this {@code UUID}.  The resulting
     * timestamp is measured in 100-nanosecond units since midnight,
     * October 15, 1582 UTC.
     *
     * <p> The timestamp value is only meaningful in a time-based UUID, which
     * has version type 1.  If this {@code UUID} is not a time-based UUID then
     * this method throws UnsupportedOperationException.
     *
     * <p>
     *  与此UUID关联的时间戳值。
     * 
     *  <p> 60位时间戳值由此{@code UUID}的time_low,time_mid和time_hi字段构成。从1582 UTC的10月15日午夜起,以100纳秒为单位测量所得到的时间戳。
     * 
     *  <p>时间戳值仅在基于时间的UUID(其版本类型为1)中有意义。如果此{@code UUID}不是基于时间的UUID,则此方法会抛出UnsupportedOperationException。
     * 
     * 
     * @throws UnsupportedOperationException
     *         If this UUID is not a version 1 UUID
     * @return The timestamp of this {@code UUID}.
     */
    public long timestamp() {
        if (version() != 1) {
            throw new UnsupportedOperationException("Not a time-based UUID");
        }

        return (mostSigBits & 0x0FFFL) << 48
             | ((mostSigBits >> 16) & 0x0FFFFL) << 32
             | mostSigBits >>> 32;
    }

    /**
     * The clock sequence value associated with this UUID.
     *
     * <p> The 14 bit clock sequence value is constructed from the clock
     * sequence field of this UUID.  The clock sequence field is used to
     * guarantee temporal uniqueness in a time-based UUID.
     *
     * <p> The {@code clockSequence} value is only meaningful in a time-based
     * UUID, which has version type 1.  If this UUID is not a time-based UUID
     * then this method throws UnsupportedOperationException.
     *
     * <p>
     *  与此UUID关联的时钟序列值。
     * 
     *  <p> 14位时钟序列值由此UUID的时钟序列字段构成。时钟序列字段用于保证基于时间的UUID中的时间唯一性。
     * 
     *  <p> {@code clockSequence}值仅在基于时间的UUID(其版本类型为1)中有意义。
     * 如果此UUID不是基于时间的UUID,则此方法将抛出UnsupportedOperationException。
     * 
     * 
     * @return  The clock sequence of this {@code UUID}
     *
     * @throws  UnsupportedOperationException
     *          If this UUID is not a version 1 UUID
     */
    public int clockSequence() {
        if (version() != 1) {
            throw new UnsupportedOperationException("Not a time-based UUID");
        }

        return (int)((leastSigBits & 0x3FFF000000000000L) >>> 48);
    }

    /**
     * The node value associated with this UUID.
     *
     * <p> The 48 bit node value is constructed from the node field of this
     * UUID.  This field is intended to hold the IEEE 802 address of the machine
     * that generated this UUID to guarantee spatial uniqueness.
     *
     * <p> The node value is only meaningful in a time-based UUID, which has
     * version type 1.  If this UUID is not a time-based UUID then this method
     * throws UnsupportedOperationException.
     *
     * <p>
     * 与此UUID关联的节点值。
     * 
     *  <p> 48位节点值由此UUID的节点字段构成。该字段旨在保存生成此UUID的机器的IEEE 802地址,以保证空间唯一性。
     * 
     *  <p>节点值仅在基于时间的UUID(其版本类型为1)中有意义。如果此UUID不是基于时间的UUID,则此方法将抛出UnsupportedOperationException。
     * 
     * 
     * @return  The node value of this {@code UUID}
     *
     * @throws  UnsupportedOperationException
     *          If this UUID is not a version 1 UUID
     */
    public long node() {
        if (version() != 1) {
            throw new UnsupportedOperationException("Not a time-based UUID");
        }

        return leastSigBits & 0x0000FFFFFFFFFFFFL;
    }

    // Object Inherited Methods

    /**
     * Returns a {@code String} object representing this {@code UUID}.
     *
     * <p> The UUID string representation is as described by this BNF:
     * <blockquote><pre>
     * {@code
     * UUID                   = <time_low> "-" <time_mid> "-"
     *                          <time_high_and_version> "-"
     *                          <variant_and_sequence> "-"
     *                          <node>
     * time_low               = 4*<hexOctet>
     * time_mid               = 2*<hexOctet>
     * time_high_and_version  = 2*<hexOctet>
     * variant_and_sequence   = 2*<hexOctet>
     * node                   = 6*<hexOctet>
     * hexOctet               = <hexDigit><hexDigit>
     * hexDigit               =
     *       "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
     *       | "a" | "b" | "c" | "d" | "e" | "f"
     *       | "A" | "B" | "C" | "D" | "E" | "F"
     * }</pre></blockquote>
     *
     * <p>
     *  返回表示此{@code UUID}的{@code String}对象。
     * 
     *  <p> UUID字符串表示形式如此BNF所述：<blockquote> <pre> {@code UUID = <time_low>" - "<time_mid>" - "<time_high_and_version>" - "<variant_and_sequence>。
     * <node>
     *  time_low = 4 * <hexOctet> time_mid = 2 * <hexOctet> time_high_and_version = 2 * <hexOctet> variant_a
     * nd_sequence = 2 * <hexOctet> node = 6 * <hexOctet> hexOctet = <hexDigit> <hexDigit> hexDigit ="0" "1"
     * | "2"| "3"| "4"| "5"| "6"| "7"| "8"| "9"| "a"| "b"| "c"| "d"| "e"| "f"| "A"| "B"| "C"| "D"| "E"| "F"}
     *  </pre> </blockquote>。
     * 
     * 
     * @return  A string representation of this {@code UUID}
     */
    public String toString() {
        return (digits(mostSigBits >> 32, 8) + "-" +
                digits(mostSigBits >> 16, 4) + "-" +
                digits(mostSigBits, 4) + "-" +
                digits(leastSigBits >> 48, 4) + "-" +
                digits(leastSigBits, 12));
    }

    /** Returns val represented by the specified number of hex digits. */
    private static String digits(long val, int digits) {
        long hi = 1L << (digits * 4);
        return Long.toHexString(hi | (val & (hi - 1))).substring(1);
    }

    /**
     * Returns a hash code for this {@code UUID}.
     *
     * <p>
     *  返回此{@code UUID}的哈希码。
     * 
     * 
     * @return  A hash code value for this {@code UUID}
     */
    public int hashCode() {
        long hilo = mostSigBits ^ leastSigBits;
        return ((int)(hilo >> 32)) ^ (int) hilo;
    }

    /**
     * Compares this object to the specified object.  The result is {@code
     * true} if and only if the argument is not {@code null}, is a {@code UUID}
     * object, has the same variant, and contains the same value, bit for bit,
     * as this {@code UUID}.
     *
     * <p>
     *  将此对象与指定的对象进行比较。
     * 结果是{@code true}当且仅当参数不是{@code null},是一个{@code UUID}对象,具有相同的变体,并且包含与此{@code null}相同的值位,代码UUID}。
     * 
     * 
     * @param  obj
     *         The object to be compared
     *
     * @return  {@code true} if the objects are the same; {@code false}
     *          otherwise
     */
    public boolean equals(Object obj) {
        if ((null == obj) || (obj.getClass() != UUID.class))
            return false;
        UUID id = (UUID)obj;
        return (mostSigBits == id.mostSigBits &&
                leastSigBits == id.leastSigBits);
    }

    // Comparison Operations

    /**
     * Compares this UUID with the specified UUID.
     *
     * <p> The first of two UUIDs is greater than the second if the most
     * significant field in which the UUIDs differ is greater for the first
     * UUID.
     *
     * <p>
     *  将此UUID与指定的UUID进行比较。
     * 
     * 
     * @param  val
     *         {@code UUID} to which this {@code UUID} is to be compared
     *
     * @return  -1, 0 or 1 as this {@code UUID} is less than, equal to, or
     *          greater than {@code val}
     *
     */
    public int compareTo(UUID val) {
        // The ordering is intentionally set up so that the UUIDs
        // can simply be numerically compared as two numbers
        return (this.mostSigBits < val.mostSigBits ? -1 :
                (this.mostSigBits > val.mostSigBits ? 1 :
                 (this.leastSigBits < val.leastSigBits ? -1 :
                  (this.leastSigBits > val.leastSigBits ? 1 :
                   0))));
    }
}
